I used the afternoon today to play around with the newly-enabled HEVC/H.265 video on newer iOS devices to compare file sizes, quality, etc.
My guess is that Apple had a hardware-accelerated chip for encoding/decoding in the iPhones 7 from the get-go and just wanted to wait for support on the macOS side for playback and viewing. This is because software encoding of HEVC/H.265 video is, at least on Intel processors, incredibly slow. For reference, here's a peek at my current editing machine:
This iMac, which is no slouch in the hardware department, can only encode between 4-7 frames per second of HEVC. Contrast that with the current standard of H.264 video, which is hardware accelerated with Intel's QuickSync, at around 8X that number. It's possible that Apple's A10 chips in newer iPhones have special instructions for encoding/decoding HEVC, but it certainly seems like a job best suited for hardware. REGARDLESS...
HEVC's main selling point is better quality and smaller file sizes through very efficient encoding methods and I've found that to be entirely true, especially when filming in 4K. In the low-movement opening scene of the video below, most of which is the sky, 30 continuous seconds of 4K HEVC footage clocked in at 173.2MB. The "More Compatible" H.264 scene was 328.4MB, just shy of double the size.
As for the quality, I think it looks great. I shot this all in HEVC and converted to ProRes HQ which, for all intents and purposes, maintains the exact visual fidelity of the original recording. I dropped it right in the timeline, added a music track, and trimmed the beginnings and ends of the clips to remove camera shake from pushing the record/stop button. So as not to introduce any further degradation, the export from FCP X was a 6.1GB ProRes HQ file that I uploaded straight to YouTube, so it's as close to the original footage as we'll get.
I tried to get shots where details might fall apart, so mostly scenes with high contrast and fine movements. I'm pretty pleased with the results right off the camera and it would look even better after a color-grading pass.
And that 6.1GB ProRes file? For a real test I took that and ran it through Handbrake's H.265 encoder with a constant quality of RF 18, which most pirates will agree produces a very close-to-Blu-Ray-quality copy of source material. The resulting video file was 241.9MB and looked really damn good. When High Sierra drops next week and brings with it native H.265 support in macOS, I'll layer the exported H.265 file over the ProRes HQ edit on the timeline, set the blend mode to Difference, and see if there's any kind of degradation at all. My gut tells me there will be some potential color shifts, but far less than anticipated.
Overall, getting a free upgrade to HEVC/H.265 will be beneficial for everyone. We'll get higher quality videos at all resolutions that end up taking up less storage space on our devices, and that's always a good thing.